This invention relates to direct current electric motors and, more particularly, to brush cooling systems for high horsepower, forced air ventilated electric motors.
The horsepower output of a dynamoelectric machine is generally derated when machine operating temperature exceeds some predetermined value. One particular area that is susceptible to higher temperature derating is the brush and commutator area of a direct current machine such as a DC motor. An aggravating factor in brush heating is that motor operation at high power output requires generally high current which increases the heating of the brush. In many high horsepower motors, some form of forced air ventilation is used to dissipate heat from the motor. The motor may be self-ventilated using some form of fan attached to a rotor shaft of the motor or may be ventilated externally by coupling a duct between the motor and a source of clean outside air. In general, air enters about one end of the motor, flows axially over the commutator, brushes, and risers and then over the armature windings, exiting at a drive end of the motor. However, in some motor designs and applications, air cannot enter from the commutator end and must be passed first through the armature before flowing over the commutator and brushes, resulting in the air reaching the brushes at 50.degree.-60.degree. C. hotter than the incoming ventilation air, thereby derating the motor.
U.S. Pat. No. 3,731,121 describes a motor of the type in which cooling air is injected from a drive end and of a type for which the present invention is particularly adapted. The motor is implemented in an electrically powered traction wheel in which the magnet frame or stator of an electric motor is made cylindrical and the wheel is rotatably mounted on the magnet frame such that the frame acts as a non-rotating axle for the wheel. The magnet frame has one end attached to or integral with a mounting flange which is used to secure the frame to a vehicle body.
The unusual manner in which the motor is mounted and forms the hub of the wheel does not allow use of conventional methods of introducing air to the motor for ventilating thereof. Maintenance and accessibility of the motor, particularly the current collector assembly and mechanical brakes, is best facilitated by placing them on the outboard side of the wheel. However, due to the semi-enclosed nature of the machine it is desirable to provide a forced ventilation system which is most conveniently applied from the inboard side of the wheel. The air flow path of such an arrangement passes axially through the motor, between the stator and the armature surface, over the commutator portion of the rotor, and out an opening in the outboard side of the wheel. When the air current reaches the commutator risers, the sudden step down to the smaller diameter commutator causes a disruption of air flow and prevents the air from passing in contiguous relationship with the commutator surface. The low pressure area which is created causes a decrease in air flow velocity and heat dissipation. Heat caused by brush friction, contact resistance losses and other factors is absorbed rather than being dissipated and the commutator temperatures increase to lower the rating of the motor.
The invention of U.S. Pat. No. 3,731,121 employs a baffle in the form of an annular air deflector secured to the inner wall of the stator or magnet frame at an axial position circumscribing the riser portion of the rotor between the armature or stator windings and the adjacent end of the commutator. Air currents traveling axially across the armature surface are deflected by the baffle toward the commutator to provide increased air velocity over the commutator surface and the brushes to increase the heat dissipation from the commutator and reduce brush temperature. The motor rating is increased by such cooling.
It will be recognized that the ventilating air flowing over the commutator in the above mentioned patent improves cooling by increased flow velocity and more assured contact with the commutator surface. The ventilating air is still flowed through the motor and therefore incurs a temperature rise of about 50.degree. C. before reaching the commutator. In order to improve the power rating of such a machine, it would be desirable to provide a method and apparatus for cooling the commutator and brushes with lower temperature ventilating air and to further increase the flow of air at the brush-commutator interface.